Leptospirosis is the most widespread zoonotic disease worldwide, and is emerging as a significant infectious disease in urban slums, particularly in tropical regions. Several species of the genus Leptospira can cause infection, which can vary in severity from mild illness to fatal hemorrhagic disease with multiple organ failure. There is a significant gap in our understanding of the pathogenic mechanisms of Leptospira species, but adhesion to host cell surfaces, extracellular matrix (ECM) components, and soluble host molecules is likely to be critical for Leptospira species to disseminate to virtually any tissue, persistently colonize certain tissues, and cause endothelial damage. In particular, binding of pathogenic Leptospira species to specific host cell- surface receptors is likely to be critical to the ability of the bacteria to cause disseminated infection and disease in immunocompetent hosts. Adhesion to cell surface molecules may also affect how the host cell(s) in the immediate environment respond to the presence of the pathogen, and how the pathogen invades tissues. At this point, however, little is known about how pathogenic Leptospira cells interact with host cell surfaces. We found that L. interrogans binds to endothelial cell surface receptors more efficiently than to ECM, and that VE-cadherin participates in this activity. VE-cadherin is the major mediator of endothelial integrity. We also identified several cell-specific adhesins based on selection for binding to endothelial cells, two f which bind to cadherins. In addition, we found that direct L. interrogans binding to endothelial cells causes disruption of the cell monolayers, concomitant with changes in the actin cytoskeleton. This leads to increased endothelial permeability to the bacteria, and likely reflects the widespread endothelial damage that is part of the pathophysiology of leptospirosis. We now propose to test the hypothesis that specific L. interrogans adhesin- VE-cadherin interactions mediate bacterial attachment to endothelial cells, resulting in disruption of the endothelium. In Aim 1, we will test the hypothesis that L. interrogans uses specific protein adhesins that recognize VE-cadherin to mediate attachment to, and disruption of, endothelial layers. We will further evaluate our candidate adhesins in several ways, including determining whether they confer increased mammalian cell adhesion activity to non-pathogenic L. biflexa. In Aim 2, we will determine the mechanism of endothelial disruption by L. interrogans using biochemical, cell biological and genetic approaches. The roles of the cell- binding bacterial adhesins we have identified in endothelial layer disruption will be evaluated. The work proposed here will significantly advance our understanding of how pathogenic Leptospira species interact with host cell surface receptors to cause infection and how these interactions contribute to the pathophysiology of disease, may illuminate new candidates for vaccine development or therapeutics.